1. Field of the Invention
The invention relates to a process for the decontamination of microlithographic projection exposure devices having optical elements, or of portions thereof, and, more particularly, of the surfaces of optical elements, with UV light and fluid. The invention also relates to a microlithographic projection exposure device with a DUV (deep ultraviolet) excimer laser as the light source of the projection exposure. The wavelength region, about 100-300 nm, is thus included in the vacuum UV.
Impurities of substrates, e.g., quartz and calcium fluoride, on the surface become extremely noticeable due to absorption during the operation of microlithographic projection exposure devices in the deep ultraviolet region (193 nm). This can cause absorption losses of up to 5% per optical element. Such absorption losses are not acceptable, particularly for semiconductor objectives. Furthermore, quartz rods or CaF2 rods are arranged in illumination devices for semiconductor objectives, to provide thorough mixing of the radiation emitted from the light source. Thorough mixing is attained by multiple total reflection of the light introduced into the glass rod or CaF2 rod. If the surface of the quartz rod or CaF2 rod is contaminated, absorption losses also occur there during the total reflection and lead to a weakening of the resulting illumination intensity.
2. Discussion of Relevant Art
It is known from U.S. Pat. No. 4,028,135 to clean contaminated quartz resonators and wafers with DUV light and a gas stream, particularly ozone. The light source used for the decontamination is arranged together with the surface to be cleaned in an aluminum box, the surface of which is a good reflector for UV light.
A process is described in U.S. Pat. No. 5,024,968 for the cleaning of optical components, particularly for X-ray lithography and UV excimer laser optics, the energy source for this purpose being a highly energetic radiation with a laser in combination with a flushing gas which is inert with respect to the surface. The cleaning is provided in this case for optical lenses and mirrors as individual components, such are dealt with during production.
An apparatus for microlithography is known from U.S. Pat. No. 4,820,899, and includes several lasers for the production of radiation for the projection exposure operation without interruption of the ray flux. For this purpose, the light or radiation emitted from pulsed lasers is coupled via an optical system into a projection exposure device, the radiation emitted from the respective instantaneously emitting laser(s) being coupled into the system by switching the optical system.
An apparatus for electron beam lithography is known from European Patent Document EP 297 506 A2. In this apparatus, the mask is cleaned by UV light sources which are provided for cleaning the mask, the electron beam being out of operation during the cleaning process. Different lasers, particularly krypton fluoride and xenon chloride, are provided for the decontamination of particles of different degrees of difficulty.
It is known from U.S. Pat. No. 5,430,303 that ions are produced by UV radiation. Absorption surfaces which can be exchanged are provided for the absorption of these ions. Furthermore, it is provided to carry out a nitrogen gas flushing in order to prevent particles depositing on the surface of the optical elements. It is known from European Patent Document EP 564 264 A1 to use the I line and G line of a mercury vapor lamp for microlithography.
It is known from U.S. Pat. No. 4,028,135 that a good surface cleaning can be attained by UV irradiation with simultaneous ozone flushing. It is known to provide irradiation with wavelengths of about 200 nm, particularly 184.9 nm, for the production of the ozone, and to provide radiation of about 300 nm, particularly 253 nm, for the particle removal. This cleaning process is known in particular for the cleaning of a quartz resonator before it is built into a projection exposure device.
However, the decontamination of microlithographic projection exposure devices in later operation remains a problem. Cleaning is only insufficiently attained with the DUV illumination used for exposure. Furthermore, cleaning with a UV source has heretofore been considered to be a problem, since the danger was seen of damage to coatings and materials.